The present invention is directed to a pressure sensor and a method of fabricating the same, and in particular to an electrical capacitance sapphire diaphragm pressure sensor for sensing a fluid pressure in, for example, a container for chemicals, a pipe for chemicals or the like, and a method of fabricating the same.
Conventional pressure sensors for sensing a fluid pressure in a container for chemicals, a pipe for chemicals or the like, are provided with a diaphragm which acts as a pressure-sensing element, whereby deflection of the diaphragm in response to an applied pressure is converted into an electric signal, to thereby sense a pressure.
Japanese Patent Application No. 2002-130442 discloses an example of such a diaphragm pressure sensor in the invention titled “Electrical capacitance diaphragm pressure sensor”.
Such a diaphragm pressure sensor normally comprises: a pressure-sensing element provided with a pressure receiving part including strip-shaped or rectangular flat plate-shaped diaphragms provided in opposing relation and deposition electrodes formed on opposing surfaces of the diaphragms; a housing element for enclosing the pressure receiving part of the pressure-sensing element, the housing element being made of a material which is resistant to corrosion by a fluid whose pressure is to be detected by the sensor; and an electronic circuit for detecting deflection of the diaphragms.
Such a diaphragm pressure sensor as described above is constituted such that when immersing a housing element in a fluid whose pressure is to be measured, the fluid pressure acts on a pressure receiving part, and the resulting variations in a distance between the opposing diaphragms cause a change in capacitance.
In a conventional diaphragm pressure sensor such as that described above, a pressure transfer coefficient varies according to a temperature of a fluid whose pressure is to be measured, and instability such as temperature drift and the like is thereby caused, and as a result, measurement accuracy is significantly compromised. It is known that a leading cause of temperature drift in a diaphragm pressure sensor is a thermal expansion/contraction coefficient of a diaphragm material.
With a view to preventing temperature drift from disadvantageously affecting measurement by a diaphragm pressure sensor, a conventional diaphragm pressure sensor, especially a metal diaphragm pressure sensor, employs a temperature compensation circuit in a pressure sensing circuit for sensing a pressure deflection of a diaphragm or disposes a temperature sensor in a diaphragm to measure a temperature of the diaphragm and provide a compensation electric signal commensurate with the thus measured temperature to a pressure sensing circuit to thereby compensate for temperature drift, that is, a thermal expansion/contraction coefficient of a diaphragm material in accordance with a temperature.
As a pressure-sensing element, a sapphire diaphragm pressure sensor in which a diaphragm is made of a sapphire plate is also known. Since sapphire has a considerably smaller thermal expansion coefficient compared to metallic materials, it can compensate for temperature drift effectively.
However, it is extremely difficult to tightly secure a sapphire diaphragm on a metal base for holding the diaphragm during the process of manufacturing of a sapphire diaphragm pressure sensor, and practical implementation of a sapphire diaphragm pressure sensor is therefore difficult.
There are a few known ways to secure a sapphire diaphragm on a metal base, such as the following:
According to a first method of securing a sapphire diaphragm on a metal base, a sapphire diaphragm is fixed on a metal base using an adhesive. According to this method, however, application of an insufficient amount of adhesive, deterioration of applied adhesive, weak binding between molecules or the like may result in a diaphragm becoming detached from the metal base. Additionally, the adhesive employed may restrict types of medium whose pressure the sensor can measure.
According to a second method, a thin layer of metal is employed to cover a diaphragm and a metal base including its fixtures. However, since a thin layer of metal also covers a surface of a diaphragm on which a pressure acts, thermal expansion of the thin metal layer covering the diaphragm causes temperature drift.
According to a third method, a sapphire diaphragm is contained in a housing element and sealed in a fluid such as silicone or the like. However, thermal expansion of the housing element as well as the fluid causes temperature drift.
According to a fourth method, which is applicable to a circular diaphragm, an O-ring may be employed to secure the diaphragm on the base. According to this method, however, a diaphragm cannot be secured in an entirely stable manner, and a leakage of a medium whose pressure is to be measured is thereby caused. Further, since a medium whose pressure is to be measured tightens the O-ring when the pressure detection begins, detection accuracy drops due to initial deflection of the diaphragm. Still further, media whose pressure can be measured by a sensor are restricted according to the material of which an O-ring is made, and a medium whose pressure is to be measured might leak if a back-up ring is not properly installed.